1. Technical Field
The present invention relates to a differential unit.
2. Related Art
Generally, in a drive system for an automobile, a differential unit is used in order to change a transmitting direction of a drive power by ninety degrees, and to carry out a final gear reduction and a differential function.
In the differential unit of the related art, as shown in FIG. 6, the differential unit 101 is provided with a drive pinion shaft 109 and a drive pinion (bevel pinion) meshing with a driven gear (not shown) at one end in a case 105 formed by a differential carrier 103 and a differential cover (not shown). The driven gear is rotatably supported in the differential carrier 103 by a differential mechanism (not shown) attached to the driven gear and by bearings (not shown), and mechanically connected to drive axles (not shown) for left and right wheels through the differential mechanism.
The drive pinion shaft 109 is rotatably supported on the differential carrier 103 by three bearings 113, 115 and 117. A companion flange 119 for linking the drive shafts (not shown) such as propeller shafts is fixed to another end of the drive pinion shaft 109 by spline fitting by using a nut 121. Cylindrical spacers 123 and 125 are interposed between inner races 113a, 115a and 117a of the three bearings 113, 115 and 117 as rotatably mounted on the drive pinion 109 in order to regulate attachment positions of the respective bearings 113, 115 and 117. The spacers 123 and 125 integrally rotate with the drive pinion shaft 109 interposed between the bearings 113, 115 and 117.
However, in the differential unit of the related art, an internal diameter of the spacers 123 and 125 is set to a larger value than that of the outer diameter of the drive pinion shaft 109 because the spacers 123 and 125 are rotatably mounted on the drive pinion 109. For this reason, the rotational axis of the drive pinion shaft 109 does not match exactly with the longitudinal axis of the spacers 123 and 125. And when the spacers 123,125 are interposed between the bearings 113, 115 and 117, there is a possibility that the rotational axis C1 of the drive pinion shaft 109 does not match the longitudinal axis C2 of the spacers 123 and 125.
If the spacers are interposed between the bearings in this way, the longitudinal axis of the spacer is offset with the rotational axis of the drive pinion shaft, and the spacers become unbalanced components and imbalance arises with respect to the drive pinion shaft. For example, in the ease where the mass of a spacer is set to 300 g and a difference between the internal diameter of the spacer and the external diameter of the drive pinion shaft is set to 1 mm, the maximum radial offset between the rotational axis of the drive pinion shaft and the longitudinal axis of the spacer is 0.5 mm, and a maximum imbalance of 15 gcm arises.
Because the spacers rotate integrally with the drive pinion shaft, vibrations will arise in the differential unit due to the above described imbalance if the drive pinion shaft rotates, and these transmission vibrations affect vehicle body vibrations.